<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>
    结果：<span></span>
</div>


<script>
    function f(x) {
        let i1 = document.querySelector("#i1");
        let i2 = document.querySelector("#i2");
        let result = document.querySelector("span");
        if (isNaN(i1.value)||isNaN(i2.value)){
            result.innerText = "输入错误！"
            return;
        }
        switch (x){
           case 1 : result.innerText = parseFloat(i1.value)+parseFloat(i2.value);
               break;
           case 2 : result.innerText = i1.value-i2.value;
               break;
           case 3 : result.innerText = i1.value*i2.value;
               break;
           case 4 : result.innerText = i1.value/i2.value;
               break;
        }


    }

</script>
</body>
</html>